In tightening up a shear bolt having a tip which is sheared when subjected to a definite tightening torque, the bolt is pretightened, before being tightened up, with a torque predetermined in accordance with a required torque. A stabilized relation between the pretightening torque and the axial tension is necessary at all times to assure the bolt of a stabilized axial tension.
The shear bolt is in a free state when merely inserted into a bolt hole since the bolt hole is sized with an allowance of about 2 mm relative to the bolt diameter.
Accordingly, it is basic practice to tighten the nut into intimate contact with the member to be fastened before the shear bolt is pretightened, whereas it is likely that this procedure will not be followed or will be insufficient.
The bolt will turn about its own axis if pretightened when the nut is not in intimate contact with the member to be fastened, namely when the nut is free or nearly free.
Further when oil or water ingress in between the bolt head and the member to be fastened, the frictional force between the contact faces of the bolt and the member becomes smaller than the frictional force between the nut and the washer, consequently permitting the rotation of the bolt about its axis during tightening.
To ensure a stabilized relation between the tightening torque and the axial tension, it is important to effect tightening by rotating the nut only without permitting rotation of the bolt and washer relative to the member to be fastened. However, the rotation of the bolt about its axis, if occurring as stated above, disturbs the relation between the tightening torque and the axial tension, so that the required axial tension becomes no longer available.
During tightening, the reaction of tightening produces a force acting to turn the tightening device around, placing a great burden on the worker who is holding the device and also entailing a hazard. This becomes more pronounced if the device is adapted for high-speed rotation.
An object of the present invention is to provide a nut tightening device which gives a proper axial tension when tightening a nut as screwed on a bolt and which is minimized in the force resulting from the reaction and acting to cause the device itself to turn around.